1. Field of the Invention
The present invention relates to a process for the preparation of phosphazene polymers. More specifically, the present invention relates to a method for catalytically producing polyphosphazenes.
2. Description of the Prior Art
The preparation of polyphosphazenes has generally been recognized to be most readily accomplished by the techniques of Allcock et al and as disclosed in U.S. Pat. No. 3,370,020. The preparation essentially involves the use of the halogenated cyclic trimer, hexachlorocyclotriphosphazene, as the starting material in what is substantially a melt polymerization process. Purified trimer is thermally polymerized under sealed tube conditions at about 250.degree. C. for 20 to 48 hours to yield a liner poly(dichlorophosphazene). The higher halogenated cyclic phosphazenes, such as the tetramer and the like, are also effective in this reaction. While the linear poly(dichlorophosphazene) itself is a good elastomer having high bimodal molecular weight, e.g. over one million, it suffers the severe disadvantage of undergoing relatively facile hydrolytic cleavage of the P--Cl bond, followed by degradation of the polymer. The prior art has shown that attempts to increase the stability of the dichloropolymer by continued heating have proved ineffective, a highly crosslinked rubbery material being produced, such also being hydrolytically unstable. Recent success for obtaining polyphosphazenes of good hydrolytic stability has been achieved by substituting all of the halogen on the linear halophosphazene polymer produced from the thermal polymerization of the trimer with various organic species. The following scheme discloses the state of the prior art to date in which high molecular weight polyphosphazenes also of bimodal distribution are produced by treating high molecular weight poly(dichlorophosphazene) I, with a variety of organic nucleophiles, e.g. alcohols, phenols, and amines, in order to obtain the corresponding completely substituted, hydrolytically stable polymers II, III and IVA and IVB. ##STR1##
Practically, the time period for accomplishing the thermally-induced, ring-opening polymerization is economically disadvantageous, and considerable effort has been expended ascertaining what catalysts could be employed to successfully promote such reaction. A variety of investigators have found that carboxylic acids, ethers, ketones, alcohols, nitromethane and metals such as zinc, tin or sodium, enhance the rate of polymerization of the cyclic trimer. See for example, German Pat. No. 2,517,142. The rate of enhancement with carboxylic acid catalysts, for example, is such that extensive polymerization is induced in 24 hours at 210.degree. C., compared to only 3% conversion to the linear halopolymer in the same time in the absence of any catalyst. Comparable catalytic activity has also been shown by sulfur (at 215.degree.-254.degree. C.), by dialkyl paracresols and by quinone or hydroquinone. See Allcock, "Phosphorus Nitrogen Compounds", Academic Press 1972, page 316 and following. Recently, alkoxide catalysts have been disclosed to be particularly effective in the catalysis of the ring-opening reaction of the cyclic halophosphazenes, giving bimodal, low molecular weight linear poly(dichlorophosphazenes) in as little as five hours at 250.degree. C. See copending application Ser. No. 731,745, filed Oct. 6, 1976.
In attempts to ascertain the types of catalysts that would facilitate thermally-induced, ring-opening polymerization of the cyclic halogenated phosphazene, a number of studies have been conducted that have been designed to establish a mechanism for this thermally-induced polymerization process. The mechanism proposed for this reaction leads to the conclusion that reagents for facilitating the removal of a chloride ion from the phosphorus atom in the cyclic halogenated phosphazene should be active catalysts. However, a variety of compounds, including those that should be good chloride acceptors, have been found to have no effect on the ring-opening polymerization and include carbon tetrachloride, chloroform, ligroin, benzene, biphenyl, cyclohexane, ethylbromide, phosphorus pentachloride, ammonia, water, mercuric chloride, aluminum chloride, and stannic chloride.
There is, therefore, a need for effective catalysts to produce linear phosphazenes.